This invention relates to utilizing the electrostatic field of a single heavy atom, or of a row of atoms extending through a thin crystal, as a lens for focusing a high-energy electron beam. Such xe2x80x9catomic focusersxe2x80x9d, having focal lengths of a few nanometers, may be incorporated in the electron-optical systems of electron microscopes so as to give a considerable enhancement of the electron microscope resolution.
The electrostatic field of an isolated atom is the positive field of the nucleus, screened by the electron cloud, and so is a smoothly decreasing positive potential which can deflect a charged-particle beam. For negative particles it can act as a positive, convergent, lens and for positive particles it can act as a negative, divergent lens. In order to be effective and useful, the position of the atom lens must not vary, i.e., it should not be moveable. One way of insuring that is the case is by holding the atom lens firmly in place by bonding it to other atoms of a solid. However, in that case the electrostatic field then ceases to decrease smoothly for distances greater than, typically, 0.1 to 0.2 nm. The atom-lens, or atomic focuser, must then be used in conjunction with an associated particle-optics device capable of focusing a beam of appreciable intensity down to these dimensions. Currently this can be achieved only for electron beams of reasonably high energy, greater then about 50 keV, by use of electron microscope systems.
The resolution of electron microscopes is currently limited to between 0.1 and 0.2 nm by the unavoidable aberrations of the objective lens. The most important aberration in this respect is the third-order spherical aberration which has a magnitude approximately equal to the focal length of the lens. This presently attainable resolution is sufficient for the imaging of single heavy atoms on suitable light-atom supports and for the imaging of the arrangements of atoms in many crystals when those crystals are held in suitable orientations with the incident electron beam parallel to a principal crystal axis. However, this resolution is not sufficient for the imaging of the atoms in crystals when the crystals have other orientations, desirable for particular purposes, or for some types of crystals even in principal orientations. For many types of investigations, and particularly when it is desired to study the arrangement of atoms in and around crystal defects, or the arrangement of atoms in poorly-ordered solids and in small particles, a resolution limit of 0.05 nm or less is highly desirable.
Resolutions approaching 0.1 nm are now possible only with high-voltage microscopes, operating with an accelerating voltage of 1 MV or more. However, for such high voltages, the high-energy electron beam produces severe radiation damage effects in most materials, making it difficult or impossible to reveal atom positions, especially around crystals defects. It would be most valuable if an improvement of resolution to better than 0.1 nm could be made for microscopes operating with electron energies of 100 keV to 200 keV, for which the radiation damage problem for many materials is much less serious and the current resolution limit is about 0.2 nm, or for microscopes with even lower energies.
One possible means for improving the resolution of electron microscopes is to use lenses of very small focal length and correspondingly small spherical aberration constants. However, it is not feasible to reduce the focal length of the commonly used electromagnetic lenses any further because of the mechanical requirements for the insertion and manipulation of specimens and the limitations on magnetic field strength imposed by the available magnetic materials.
There are several papers in the reported literature which incorporate the idea that high-resolution images of arrangements of atoms may be achieved by taking advantage of the fact that electrons may be concentrated into beams of very small diameter by transmission through thin crystals. These papers refer to the channeling of electrons through crystal lattices which gives a concentrated fine beam of electrons at each atom position of the exit face of the crystal and allow, in effect, a high-resolution imaging of atoms attached to the exit face. In a series of papers, J. T. Fourie, 90 Optik, 37, 85 and 134 (1992), and 95 Optik 128 (1994), Fourie proposed, incorrectly, that the channeling effect creates concentrations of electrons between the atom rows and gave examples of high-resolution detail of a layer of atoms sitting on the exit face of a gold crystal. Loane et al. in xe2x80x9cVisibility of Single Heavy Atoms on Thin Crystalline Silicon in Simulated Annular Dark-Field STEM Images,xe2x80x9d A44 Acta Cryst. 912-927 (1988) which is incorporated herein by reference, correctly described the channeling of electrons as being along the row of atoms in a crystal and calculated the intensities for the high-resolution images of gold atoms sitting on the exit face of a silicon crystal. None of these papers describe or suggest the use of the atoms as a lens to form small cross-overs at a distance beyond the exit face of a crystal.
Accordingly, it is an object of the present invention to improve the resolution of electron microscopes.
Another object of the invention is to provide a method for focusing an electron beam.
A further object of the invention is to provide an electrostatic lens using the electrostatic potential of an atom or atom array to focus an electron beam.
These and other objects are achieved by the present invention, which provides a method of focusing an electron beam comprising forming an electron beam probe having a beam size less than about 0.2 nm at a focal plane, and focusing the electron beam probe using a fixed atom or atom array focuser having sufficient electrostatic field at the focal plane whereby the fixed atom or atom array acts an electrostatic lens. Preferred for such focusing are heavier atoms, of elements having atomic number greater than about 20.
Included are the following schemes, techniques or configurations for an ultra-high resolution electron microscope:
(1) an electrostatic lens device comprising a fixed atom or linear atom array disposed at a focal plane of an electron beam probe having a diameter of less than about 0.2 nm at the focal plane; as well as an ultra-high resolution microscope comprising an electron beam generator for generating a beam having an energy greater than about 50 keV, an electron beam focuser for focusing the beam to about less than 0.2 nm at a focal plane, an electrostatic lens comprising a fixed atom or atom array placed in the electron beam at the focal plane, a sample holder for holding a sample in the electron beam after the electrostatic lens at a distance from the electrostatic lens which can be varied by a suitable distance adjuster device, and a detector for detecting the electron beam after passing through the sample;
(2) an ultra-high resolution microscope comprising an electron beam generator for generating an electron beam having an energy greater than about 50 keV, similar to the illumination system of a conventional transmission electron microscope, for illuminating a sample, an electrostatic lens comprising an atom or linear array of atoms aligned in the incident beam direction, a device for varying the distance between the sample and the electrostatic lens, and a dectector having a resolution limit not exceeding about 0.2 nm;
(3) an ultra-high resolution electron microscope comprising an electron beam generator for generating an electron beam having an energy greater than about 50 keV and a divergence of less than about 1 mrad, a thin crystal forming a two-dimensionally periodic electrostatic focuser, a sample holder for holding a sample in the electron beam at a Fourier image distance from the periodic electrostatic lens, a device for translating the Fourier image of the periodic electrostatic lens relative to the sample, and a detector device having a resolution limit not greater than about 0.2 nm;
(4) an ultra-high resolution electron microscope comprising an electron beam generator for generating an electron beam having an energy greater than about 50 keV, an electron beam focuser for focusing the beam to a diameter less than about 0.2 nm, a sample holder for holding a sample in the electron beam at the focal plane, a thin single crystal forming a two-dimensionally periodic electrostatic focuser for placement at a distance from the sample equal to a Fourier image distance for the periodic focuser, and a two-dimensional detector or detector array for detecting electron beam after it has passed through the periodic electrostatic focuser;
(5) an ultra-high resolution electron microscope comprising an electron beam generator for generating an electron beam having an energy greater than about 50 keV, an electron beam focuser for focusing the beam to about less than 0.2 nm diameter at a focal plane, a device for laterally scanning the beam at the focal plane in two dimensions, a sample holding for holding a sample in the focal plane of the focuser, a thin crystal forming a two-dimensionally periodic array of electron focusers, and a detector system having a resolution limit which is greater than 0.2 nm and which may be as great as 10 nm;
(6) an uitra-high resolution electron microscope comprising an electron beam generator for generating an electron beam of energy greater than about 50 keV, an electron beam focuser for focusing the electron beam to a diameter of less than about 1 nm at a focal plane, a device for scanning the beam at the focal plane in two dimensions, a thin crystal forming a two-dimensionally periodic array of electrostatic lenses, a sample holder for holding a sample within a distance of a few nanometers after the two-dimensional electrostatic focuser array, and a detector system having a resolution limit not greater than a bout 0.2 nm.